I'm considering replacing am old gas water heater (Main Medway) with an ele ctric one. As far as I can see it needs at least 18kW for a bath or shower, maybe more like 21kW or 24kW. I just need to supply a kitchen sink and a s hower/bath. Adjacent rooms, small flat.
I have the usual 240v supply, and some but not all of these units are 400v. Looks like pretty heavy gauge wiring required, like 8 gauge.
Is this practical?It's hard to get a lot of information on these kind of un its, though they're widely used in the USA and Germany.
I installed a 9kW one of these while our electric storage heaters were removed while we installed gas central heating. Easy enough to install as we had a cable in place we could use temporarily from one of the old heaters. It's a lot of power so cable must be beefy enough of course and must ensure an adequate Earth. In the event it supplied only warm water unless the flow valve was turned down very low. However that was much better that very cold water! As to using it for a shower I don't think it would be very good as the flow rate was so low through it.
23kW @ 230V is 100A, which is the max rating of a single-phase supply, so you aren't likely to find one for use in the UK.
If you need a supply of more than 23kW, you have to take a multi-phase supply, and you run large loads of this type between phases, hence 400V.
Multi-phase domestic supplies are more common in Germany than in the UK, but you can ask to be upgraded to one in the UK.
If you want to go electric, your best bet might be a 12kW shower (which probably won't feel as good as the Main Medway managed) plus a small stored water heater for the kitchen sink.
However, with gas you have a couple of options with power outputs exceeding your current Main Medway:
I think Main (now Baxi Potterton) still make instant water heaters which can directly replace your existing one, and even hang on the existing flue terminal providing it's still in good condition. They also do electric ones which don't burn a pilot light all the time (makes them much more efficient), but will need a new (fanned) flue terminal.
Second option is to replace it with a combi boiler which will do your heating and hot water, and they aren't much bigger than the Main Medway (most will be a little deeper).
But if you have gas available, why not just stick in a combi boiler or another gas water heater?
Gas is maybe 30% the price of daytime electricity and you'll get 20-30kW units without problem. That sort of power with electricity is going to need an uprated main feed - probably 3 phase as you note.
24KW is 104A at 230V, which is going to be too much on a domestic supply, the flat may not even have a 100A supply.
18KW is 78A and would require 16mm cable based on a run of 10m not in any insulation, and again, may be too much for the flat's supply.
Your standard UK supply is usually 60, 80 or 100A.
I would suggest sticking with gas here or having some hot water storage, like an immersion heater.
Gas is going to be the cheapest to run, assuming it is mains gas.
In response to questions, the flat which is basically a decent size bedroom and a decent size living room + small bedroom, is heated with two open gas fires in the main rooms. This is economical, so I'm debating whether I nee d central heating. The Main Medway is just right for filling a bath. Neglig ible needs in the kitchen - just filling a sink.
I was initially going to replace the Main Medway with a combo boiler, but I 'm very restricted as to where the flue can go out and it's on the 2nd floo r. I have very few options, and even those aren't ideal. Right now the Main Medway is in a small utility room which I'd like to use as a bedroom. I be lieve that with that type of heater you can have it in a bedroom, but pleas e tell me if regulations have changed. Combi boilers are out in bedrooms of course, and so are exit pipes from combi boilers. The layout is OK as it i s and it would be a real pain to change it, hence an electric unit would ma ke things easier.
I do use a bath, so has to be 18kW minimum and 21kW or 24kW better. I'll ha ve to find out what supply I have and if it's 100A.
Can't see the issue of having a combi in a bedroom (maybe build a cupboard around it) as long as it is installed correctly, it will be room sealed. Fit a carbon monoxide alarm too, so you know if there is a problem.
om and a decent size living room + small bedroom, is heated with two open g as fires in the main rooms. This is economical, so I'm debating whether I n eed central heating. The Main Medway is just right for filling a bath. Negl igible needs in the kitchen - just filling a sink.
I'm very restricted as to where the flue can go out and it's on the 2nd fl oor. I have very few options, and even those aren't ideal. Right now the Ma in Medway is in a small utility room which I'd like to use as a bedroom. I believe that with that type of heater you can have it in a bedroom, but ple ase tell me if regulations have changed. Combi boilers are out in bedrooms of course, and so are exit pipes from combi boilers. The layout is OK as it is and it would be a real pain to change it, hence an electric unit would make things easier.
have to find out what supply I have and if it's 100A.
You could look at some kind of heatbank cylinder. Something like this:
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is not much bigger than a boiler, and would do better hot water than a smal l combi. Something like
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would also do the centra l heating, but is a bit bigger. Both are heated electrically, and could be mainly heated overnight, and jus t topped up in the day if needed, so if you switch to a 2 rate supply, it w ould be a similar cost to run as gas.
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An electric shower will probably only need 10kW (for a fairly feeble shower it has to be said). But bath filling with instant electric is going to be an exercise only for the very patient.
Other than for hand washing etc, its probably a non starter unless you want to go to lots of hassle. An unvented cylinder can be heated by electric (and could take advantage of cheap rate power as well). That would give a much better experience for both showers and baths.
There are 3 phase ZIP & other instant water heaters. However it will cost you a fair chunk to upgrade the flat supply to 3ph.
It is possible to use an electric shower at 8.5-9.8-10.5-12.5kW. That is assuming the supply is at least 60A, preferably 80A (ie, not 40A).
However you can forget a bath re standing losses of the bath and fill time. A 9.5kW ZIP is basically 4L/min in winter at 38-40oC ... a bath is 90L. You need a low flow head - 6L/min at best, aeration or ultra low flow.
The solution is most likely a) electric shower & dedicated undersink 5L/10L stored water heater (vented/unvented) in kitchen or b) stored UNVENTED wat er heater of 50L capacity for a shower (Powerflow) or 90L for a bath.
Note stored UNVENTED are a higher temp (60-65oC) so their effective volumet ric capacity is higher than it appears re TMV (thermostatic mixer) set to s ay 38-41oC. It needs a G3 plumber to install big unvented (or notification and do the bulk of the work yourself IIRC), they MUST be installed with all the correct pressure & temp release pipework run correctly re sizing, bend s, etc. It is not a trivial task - best to go read the instructions on one online and consider the location re pressure & temp outlet pipework. Such h eaters can however be 1kW to 4kW, so work fine with a modest electrical sup ply.
Looks like the Main Medway unit I have now is the easiest solution. I was informed that such balanced flue room sealed water heaters are obsolescent and may not exist in a few years - don't know how true that is.
But at least, as far as I can tell, regulations still allow room sealed balanced flue units to be used in bedrooms so I could put a bed in the room. Let me know if that's incorrect.
*BALANCED* flue means air for combustion AND products of combustion are han dled externally to the room - just like a modern combi/standard gas central heating boiler. I see no problem with that surely?
It IS however common for small flats to have a 50/90L unvented water heater on 3kW supply for shower/bath. A 50L unit will do (size) if fitted with TM V to 38-41oC, 6L/min shower head (aerator oxygenics or something), and a se parate 5-10L unvented or 15L unvented water heater for the kitchen. The lat ter is more at the luxury end re backup if the other goes off, and 5L will still give 9L of hot water re reheat/mixed-temp time, with most likely a ta ble-top or compact dishwasher in use. Most washers being cold fill these da ys too. The 5L vented units from Stiebel Eltron do not drip from the tap (p eople keep tightening until the costly tap is stuffed) because they effecti vely have an internal expansion loop.
I would go for a) gas b) unvented 50L unit (& mini for kitchen re redundanc y) and lastly c) instant for shower only (useless for a bath).
Curiously enough there's a 3 phase supply which must have been fitted into this once-single terrace house long ago (maybe in 1970) when it had completely all-electric heating with multiple timers, room thermostats, etc. which were still there although disconnected until I completely removed them. Far ahead of its time; maybe the occupant worked for (ran?) the electricity board and got a rebate. (!!!).
It's now divided into two and, if memory is correct, the rewiring uses a different single phase for the upper and lower flats.
I wonder what it might cost to have a 3 phase meter fitted instead of the current single phase 3 rate meter (if that's possible), or what the electricity charges might then be?
With all the changes in energy costs and security of supply, what must once have been an expensive arrangement might soon beome the best. (Might be better still if energy suppliers were forbidden by law from selling both gas and electricity; why did our Glorious Leaders reduce competition by allowing this?) (Unless of course they own shares).
Didn't know they went as high as 12Kw; I've certainly found 8.5 Kw inadequate at the end of the winter when resevoir temperatures have dropped near their lowest. I would want a 10.8 Kw ~= 45A. unit if replacing.
And they can freeze in winter if shut off by the occupants of the flat above you, as I know to my cost.
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